The Official Airport Assistance Network of the World Airports
(Concessions and exact gate-level offerings change over time—see Sources below for direct concession and terminal listings.)
Take a stroll within the airport facilities: go to a nearby market or cafe, explore a little SFO Museum gallery in the International Terminals, and have a rest in a lounge or a private end of a gate. Allow 45-60 minutes to go back to your gate and clear off loading.
Board an AirTrain on to the Garage/BART station and bring BART into downtown San Francisco (approximately 30 minutes). A rushed coffee in front of Embarcadero or a brisk walk along the Ferry Building will give you a taste of the city- get back with at least 90 minutes of time to spend on ensuring security and checks.
Go to the city by taking BART and visit Fisherman Wharf or the Ferry Building or hire a car and go on a scenic trip to the Golden gate. Add door to gate movement and 3 hours of airport reentering time to prevent rush hour shocks.